1. Field of Invention
The present invention relates to wavelength-converted semiconductor light emitting devices.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
Since the light emitted by III-nitride devices is generally on the shorter wavelength end of the visible spectrum, the light generated by III-nitride devices can be readily converted to produce light having a longer wavelength. It is well known in the art that light having a first peak wavelength (the “primary light”) can be converted into light having a longer peak wavelength (the “secondary light”) using a process known as luminescence/fluorescence. The fluorescent process involves absorbing the primary light by a wavelength-converting material such as a phosphor and exciting the luminescent centers of the phosphor material, which emit the secondary light. The peak wavelength of the secondary light will depend on the phosphor material. The type of phosphor material can be chosen to yield secondary light having a particular peak wavelength.
Phosphors may be disposed in the path of light emitted by an LED in several ways. U.S. Pat. No. 6,351,069 describes a III-nitride LED die covered by a layer of a transparent resin in which a wavelength-converting material is mixed. U.S. Pat. No. 6,630,691 describes growth of LED devices on single crystal luminescent substrates. U.S. Pat. No. 6,696,703 describes the use of thin film phosphor layers disposed over LEDs. Several patents describe forming conformal phosphor layers on LEDs, for example by electrophoretic deposition as described in U.S. Pat. No. 6,576,488, or by stenciling as described in U.S. Pat. No. 6,650,044. Many of these phosphor layers are fragile and difficult to handle, and are intolerant of the high temperature and high flux environment created by the LED. In addition, it may be difficult or impossible to form multiple phosphor layers by some processes.
An alternative to the above-described phosphor layers is the use of wavelength converting materials formed into ceramic slabs, as described in more detail in U.S. Patent Application Publication 2005-0269582, which is incorporated herein by reference. The luminescent ceramic slabs described therein are generally self-supporting layers formed separately from the semiconductor device, then attached to the finished semiconductor device or used as a growth substrate for the semiconductor device. Luminescent ceramics tend to be more robust than the phosphor layers described above.